1. Field of the Invention
The present invention relates to a liquid crystal display or a liquid crystal panel and a manufacturing method of the same. More particularly, the present invention relates to a liquid crystal display consisting of a pair of substrates of which at least one is transparent and which have a medium sandwiched therebetween that has light switching function and is generally known as liquid crystal, wherein uniformity in contrast, response speed, etc., can be improved and a good display quality can be achieved by maintaining a uniform and constant gap between the substrates over the effective display area, and also relates to a manufacturing method of the same.
2. Description of the Related Art
A liquid crystal display consisting of a pair of substrates of which at least one is transparent and which has liquid crystal, that is, a medium having light switching function, filled therebetween, has several advantages in that it is generally thin and light in weight, has a low power consumption, etc. Therefore, it is widely used as a display device in electronic calculators, household appliances, or in OA (office automation) equipment. A liquid crystal display is also used in an input apparatus of an information processing system and in computer-holograms as a spatial light modulator.
A liquid crystal panel used for a display in an OA equipment has typically the construction of a pair of substrates having liquid crystal filled and sealed therebetween. On one of the substrates, a TFT (Thin Film Transistor) and a pixel electrode is formed for each pixel, and on the other substrate a common electrode is formed that is common to all pixels. Hereinafter, the substrate on which pixel electrodes and TFTs are formed will be referred to as the TFT substrate, and the substrate which is disposed opposing to the TFT substrate will be referred to as the opposing substrate.
In a liquid crystal display, it is essential, to obtain a good quality display, that the gap between the TFT substrate and the opposing substrate be uniform and constant. In order to maintain a uniform and constant gap, members are used which are disposed between the substrates and are generally called spacers. Spacers are broadly divided into bead-shaped spacers and pillar-shaped spacers, and various methods have been developed for arranging and fixing these spacers.
One method which has been proposed for uniformly controlling the gap between substrates is to scatter spherical particles (beads) on the substrate. In this method, however, it is difficult to control the distribution of beads, and beads tend to be scattered also in the area of pixels that is essential for display. This gives rise to orientation defect of the liquid crystal molecules, which leads to the problem of deterioration of the display quality.
In order to overcome this problem, a method has been proposed in which, in place of scattering beads, a photolithographic method is used to selectively form pillar-shaped spacers outside the area of pixels (for example, Japanese Patent Publication Nos. 08-220546, 2001-82517, and 2001-201750). In this case, there is no spacer in the essential pixel area, and therefore, defects in orientation and a deterioration of display quality can be avoided. However, spacers are usually adhered to only one of the substrates and are not adhered to the other substrate. Therefore, as shown in FIG. 7, when spacers 51 are adhered only to the opposing substrate 50, but not to the TFT substrate 60, if external pressure is applied to a point A between the spacers 51, cell gap around the point A may vary considerably and may give rise to interference fringes or fluctuation of color tone or of drive voltage characteristics. In extreme cases, the substrates may come into contact with each other and the orientation film may be damaged, leading to a disturbance in the orientation of liquid crystal molecules and to deterioration of display quality.
Thus, the possibility of adhering spacers to both substrates has been examined, but it has been difficult to realize both precise control of the gap and strong adherence to substrates. For example, it is proposed in Japanese Patent Publication No. 2000-155321 to include beads which are not deformed by application of pressure or heat, in the spacers, to realize both high strength and strong adhesion to substrates. However, since the beads are dispersed in resin material for forming spacers, problems arise as described below:
(1) There is a probability that some spacers do not contain beads, and this degrades the gap uniformity.
(2) If, in order to avoid above problem, the content of the beads is increased, the beads act as fillers and produce adverse effect, in that, when manufacturing spacers, spacer material cannot be spin-coated uniformly, leading to fluctuations in the coating thickness which results in non-uniform heights of the spacers.
(3) Beads with uniform size are very expensive, and since beads outside of the spacer are removed and cast off, most of the beads are wasted, leading to a high manufacturing cost.
Pillar-shaped spacers are largely composed of beads having high hardness so that they cannot follow rapid oscillation of substrates due to shock, especially expansion of the substrates, and the pillar-shaped spacers tend to separate from the substrates, or to be broken.
Further, a method is disclosed in Japanese Patent Publication No. 08-110524 in which spacers coated with thermoplastic resin are scattered on one of the substrates, and when two substrates are adhered, spacers are joined to both substrates by the thermoplastic resin. However, it is impossible to dispose the spacers in desired positions with this method.
Thus, the prior method for arranging and fixing spacers for a liquid crystal display has not been satisfactory for realizing a liquid crystal display which is resistant to an external force (highly resistant to shock) and has good display characteristics.
Further, with the prior spacers, it has been difficult to coat the solution for orientation film so as to arrange spacers uniformly and in high density on the substrate, or spacers may separate from the substrate during rubbing treatment of the orientation film. Thus, it is not satisfactory for fixing spacers on the substrate while maintaining the display quality of the liquid crystal display.
When a photolithographic method is used to control the substrate gap, using spacers which are formed and completely hardened on one of the substrates outside of the pixel region, sufficient adhesion to the other opposing substrate cannot be obtained so that the gap between electrodes at the center portion of the substrates becomes small under an external pressure, giving rise to interference fringes, fluctuation of color tone or fluctuation of the drive voltage characteristics, and, in an extreme case, upper and lower substrates may come into contact with each other and the orientation film may be damaged, leading to problems such as disturbances in the orientation of liquid crystal molecules and a deterioration of display quality.
On the other hand, it is highly desirable to form spacers from resin alone and realize both high strength and strong adhesion. Spacers can be formed from resin alone, for example, by coating the resin material for the spacer on an orientation control film formed on one of the substrates and then patterning the resin film by exposure to light and development. A spacer thus obtained adheres strongly to the orientation control film on which the resin material for the spacer was coated. However, when the spacer thus formed is superimposed on the other substrate in subsequent step to form a liquid crystal panel, it does not adhere, after being melted by heating and solidified, with same strength to the orientation control film formed on the other substrate. Although various resin materials have been examined for spacers, and development of resin material capable of improving adhesion to an orientation film is actively in progress, a novel technology which enables stronger adhesion is still to be realized.
With regard to rubbing of an orientation film, besides the problem of separation of spacers, there is another problem that debris of spacers produced when the spacers are scraped off by the brush for rubbing, may give rise to display defects in the liquid crystal display.